Sunday, March 1, 2009
How It’s Done: Turbine Engine Overhauls
What happens during an engine overhaul? Let’s walk through the engine shop and see for ourselves!
You’ve just walked into the hangar and noticed something unusual. There’s an empty space where your helicopter’s turbine engine used to be. But it’s okay. It was pulled out by the mechanic and sent to an authorized service center for a scheduled overhaul. In 20-30 days, however, it’ll be reinstalled back in its rightful place, ready to be flown for several hundred more hours. But what happens to your engine after it reaches the repair facility?
This month, Rotor & Wing will escort you through the process of a turbine engine overhaul at Sikorsky’s subsidiary, Keystone Helicopter, located in the rolling hills of Coatesville, Penn., 30 miles west of Philadelphia. Our guide will be Chuck Hurdelston, program and sales manager for the engine division.
"Here are the basic steps," said Hurdleston. "Receiving, preliminary inspection, teardown, cleaning, detailed inspection, quoting, re-work, assembly, testing, quality assurance and finally, shipping."
Engines usually arrive mounted inside a shipping box and are immediately taken into the engine shop of Keystone’s massive 250,000-square-foot facility where they also assemble, complete and repair Sikorsky S-76s and S-92s. As soon as an engine is uncrated, a set of tracking documents and work orders are placed inside of a clear plastic pouch that will move through the overhaul process with the engine, thus ensuring accountability.
"Be sure to tell your readers to make copies of the engine log book before they send the original to us," said Hurdleston. "If the originals are lost by the shipping company, it’s going to complicate matters."
After the engine has been removed from its crate, it is mounted onto a wheeled stand that allows it to be rolled to the inspection and disassembly areas easily. The rack also allows the engine to be rotated end-over-end for closer inspection and ease of dismantling.
During preliminary inspection, a technician will take a good, superficial look at the engine and its accessories, checking to see if there are any obvious problems, such as fluid leaks and corrosion. If anything out of the ordinary is found, a notation will be made on an in-house form and slipped inside of the document pouch for later action.
While on the rack, the engine will also be bled of fluids, which Keystone will send to recyclers.
At the front of the engine shop are the disassembly stations. Here, technicians methodically take the engine apart down to the smallest components. Each part is then placed on a wheeled cart with foamed cutout cradles for major pieces, such as turbine wheels, fuel controllers and shafts. Each part gets a visual check for obvious problems at this point, as well.
Once the engine has been disassembled, it can now be moved around to other locations in the shop on its wheeled, shelved cart.
Nearly everything in an engine needs some sort of cleaning during the inspection process; some because they will be returned to service aboard the customer’s aircraft and others because soot, lubricants and discoloration can hide damage during the inspection process. So, engine parts will be tagged individually with the main tracking number, and sent off to be cleaned by a process that’s appropriate for the component and the material it is made out of.
Inside of the well-ventilated cleaning room are a variety of vats and sinks where chemicals are used to clean certain parts.
"You’ve got various hot soak and cold soak solutions here," said Hurdleston as he pointed out the heavy cleaning stations, some of which looked like deep fryers from a restaurant. "The parts get agitated and cleaned in sodium hydroxide and other chemicals, to name a few. We use ultrasonic cleaning, too."
But sometimes the accumulated dirt and corrosion are so thick it has to be blasted off the part. For that, they go into the grit blast room.
Inside the grit blast room, components are placed in large metal cabinets where technicians literally blast the part clean using sprayers that attack the dirt with "grit," which is actually glass or plastic milled into a fine granular substance. When properly used, the part is left clean and virtually intact.
Once everything is clean, the parts are sent to various stations in the engine shop for detailed inspection following the procedures set forth by the engine’s manufacturer. Of particular interest are the "wheels," meaning the rotating parts that make up the compressor that packs air into the engine; the power turbine, which drives the transmission and some of the accessories; and the exhaust turbine, which helps drive the compressors in the front of the engine by way of a shaft.
Of all the part of an engine, the wheels require the most precision to build because they have to be perfectly balanced and spin freely within a hair’s width of their housings. Therefore, during an overhaul inspection, these parts must be checked for even the slightest bit of deformation, pitting, corrosion or deterioration.
In one area of the engine shop, technicians take measurements of parts to ensure they are within the manufacturer’s wear limits. To accomplish this, they use hand calipers and computer-assisted laser triangulation to identify the exact amount of wear on a part, as well as its alignment.
Balance is key to a properly running engine. If its shafts and wheels are not spinning true, the engine will vibrate, causing even more damage or catastrophic failure.
To ensure proper balance and alignment of rotating parts, an assortment of special tools are used to identify warped, worn or deformed areas where various components mate with each other. This way, a bearing seated in a deformed housing, for example, won’t impart its unwanted vibration onto another part.
In another area of the shop, a technician sits in a darkened room inspecting a compressor rear support using a process known as "dye penetrant."
For a dye penetrant inspection, the technician spreads a special powder over critical portions of the component. When examined under ultraviolet light, the powder, which will appear bright green under the UV light, will highlight any pits and cracks that the naked eye would miss. If any are discovered, the technician marks them for later evaluation.
Not far from the shop where UV light is used, another technician is mounting turbine and compressor wheels on the side of a large airflow machine. One by one, the machine will test the efficiency of compressor and turbine wheels by driving a prescribed volume of air through them at a specified velocity. A computer read-out will alert the technician to any issues that are prohibiting the proper handling of air, especially vane deformation.
The engine’s fuel system will get a detailed inspection, too. For that, there are three machines waiting in a separate room. They are the fuel nozzle flow cabinet, which allows the technician to see fuel being sprayed out of the engine’s nozzle to ensure proper volume and spray pattern; the power turbine governor test cart, which looks at the accuracy of the component’s ability to regulate fuel; and the fuel accessories test stand, which powers up the engine’s fuel controller to check its functions.
Once the various sub-shops of the engine department have competed their tests and placed their findings in the plastic document pouch that the engine was given when it arrived, a specially trained member of the administrative staff with extensive engine repair experience will review all of the identified problems with that engine. That person will then prepare an itemized estimate for the customer, indicating what needs to be repaired, what needs to be replaced, and what the estimated cost for the entire overhaul job will be. The quote will then be sent to the customer for their review.
Once the customer has approved the recommended course of action for the completion of the overhaul, workers will go about pulling the needed parts from stock, ordering items that aren’t on-hand, and, in the case of repairable components, make those items ready to be returned to service aboard the engine.
When a part that comes in with an engine has a defect that the engine’s manufacturer says can be repaired, that item is tagged for what is called "rework."
In most cases, reworking is a matter of grinding scored or pitted surfaces down until smooth, then rebuilding the affected area until it returns to the proper dimensions. This process takes two separate shops; the machine shop and the plasma spray booth.
The machine shop is equipped with an assortment of large grinding and drilling machines that carefully file away deformities in metallic surfaces much the same way a piece of wood is shaped on a lathe. Technicians command the computers on the machines to remove the required amount of material until smooth and perfectly round.
The newly filed component is then taken to the plasma spray booth, where molten material it sprayed onto the surface of the recently smoothed part. Once it has cured in a brick-lined oven, the part is returned to the machine shop where it is machined again until ground down to the manufacturer’s specifications. When completed, the part will be sturdy enough to withstand the rigors of another few hundred hours of operation.
Not everything can be repaired at every engine shop, though. Some parts – different ones for different engines – may only be reworked by the manufacturer or a very small number of factory-authorized centers. Consequently, the reworking process sometimes involves sending worn or damaged parts to another place for repair. With luck, those parts will be returned to the engine shop soon after the components done in-house are finished.
Once all of the engine’s parts have been inspected, replaced or repaired according to the manufacturer’s specifications and the owner’s wishes, it is time to assemble them back into one fully functioning power plant.
The wheeled rack of components gets a bag of brand new gaskets before being handed over to a technician for assembly. Care will be taken to torque all nuts and bolts to factory specifications as the engine is built up mounted back on a wheeled engine stand. When done, it will be pushed over to the engine test cell.
The best way to test an engine before reinstalling it aboard an aircraft is to mount it in a test cell. The engine test cell consists of two specially constructed rooms; the test cells itself and the operator’s room.
The test cell is where the engine mount is located. It is here that the engine will be bolted to a rack, connected to fuel and oil lines, and then hooked up to various probes and sensors that will monitor its performance. A dynamometer or "dyno" is attached to the output shaft of the engine to measure torque, as well as to put the engine under a load similar to what it will have to operate under in the helicopter. A ceiling vent carries engine exhaust outside where a color camera watches the chimney for trouble that may identify itself in the form of smoke or super-intense heat plumes.
A critical part of the engine test cell is a steel, rail-mounted shroud that is positioned around the turbine housing when the engine is running. In the unlikely event of a catastrophic engine failure, the shroud will minimize the danger of parts literally flying off the engine.
Adjacent to the test cell is the sound-dampened operator’s room. From here, the technician will run through a series of manufacturer-prescribed tests for a particular model engine. It usually includes running the engine at idle while a second technician conducts a visual check for leaks. If all is well, the engine will run through a range of RPMs as its performance is monitored on computer screens and compared to pass/fail parameters set down buy the designers.
If the engine fails any portion of the test, the computer will point to the general source of the trouble, and that component will be sent back for rework or replacement. If the engine passes, it will be wheeled to a holding area while its paperwork is reviewed by quality assurance personnel.
The quality assurance, or QA, people will read through the paperwork to ensure that all of the required work was properly done, carefully documented and appropriately recorded in the customer’s engine log books. Once QA gives it the thumbs-up, the engine can be readied for return to the awaiting owner.
When preparing to ship, shop technicians will ensure that the engine has enough lubricant to protect critical parts from the affects of corrosions during return shipment, since some returns will include passage aboard an ocean-going vessel. The engine is then put back into the shipping crate it arrived in, and taken to the loading dock to be sent out.
Chances are that when the worker drops a newly overhauled engine at the shipping desk to be sent out, another engine will be waiting in the receiving area for its turn to be overhauled. And the process begins again.